W.J. Porter

Effects of ballistic impact damage on fatigue crack initiation in Ti–6Al–4V simulated engine blades

Abstract The ingestion of debris into jet engines creates nicks and dents on the leading edges of blades and vanes. This is commonly known as foreign object damage (FOD). Such damage, which can often result in premature failure, was simulated in the laboratory using diamond cross-section axial fatigue samples that were impacted with 1 mm diameter glass beads at 305 m s −1 at either 0 or 30° angle of incidence. The samples had either a thin leading edge (LE) with a radius of 0.127 mm or a thick LE with a radius of 0.381 mm. Fatigue strength of impacted specimens showed degradation of 10–50% due to LE damage, regardless of the depth of the damage zone. FOD related impact notch depth, loss of material (LOM), shear, folds, embedded shattered glass, and microstructural damage were characterized by SEM. Fatigue strength degradation was found to be higher for the 30° impacts than for the 0° impacts. No clear correlation between notch depth or LE thickness and fatigue strength was found.

Determination of Young’s modulus of grains in a gamma titanium aluminide alloy

Abstract An experimental and analytical process for determining the elastic response of grains in a gamma titanium aluminide alloy was developed. Three-dimensional finite element models were employed to deduce the transversely isotropic properties. The results are compared with those reported for polysynthetically twinned materials.

Correlating Scatter in Fatigue Life with Fracture Mechanisms in Forged Ti-6242Si Alloy

AbstractUnlike the quasi-static mechanical properties, such as strength and ductility, fatigue life can vary significantly (by an order of magnitude or more) for nominally identical material and test conditions in many materials, including Ti-alloys. This makes life prediction and management more challenging for components that are subjected to cyclic loading in service. The differences in fracture mechanisms can cause the scatter in fatigue life. In this study, the fatigue fracture mechanisms were investigated in a forged near-α titanium alloy, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, which had been tested under a condition that resulted in life variations by more than an order of magnitude. The crack-initiation and small crack growth processes, including their contributions to fatigue life variability, were elucidated via quantitative characterization of fatigue fracture surfaces. Combining the results from quantitative tilt fractography and electron backscatter diffraction, crystallography of crack-initiating and neighboring facets on the fracture surface was determined. Cracks initiated on the surface for both the shortest and the longest life specimens. The facet plane in the crack-initiating grain was aligned with the basal plane of a primary α grain for both the specimens. The facet planes in grains neighboring the crack-initiating grain were also closely aligned with the basal plane for the shortest life specimen, whereas the facet planes in the neighboring grains were significantly misoriented from the basal plane for the longest life specimen. The difference in the extent of cracking along the basal plane can explain the difference in fatigue life of specimens at the opposite ends of scatter band.